FIGS. 5 and 6 illustrate a fuel cell, where a PEFC (Polymer Electrolyte Fuel Cell) 10 includes a membrane-electrode assembly (MEA) and a separator 18. The MEA includes an electrolyte membrane 11 and a pair of electrodes 14 and 17 disposed on opposite sides of the electrolyte membrane 11. The pair of electrodes 14 and 17 includes an anode 14 provided on one side of the membrane and constructed of a first catalyst layer 12, and a cathode 17 provided on the other side of the membrane and constructed of a second catalyst layer 15. A first diffusion layer 13 may be provided between the first catalyst layer 12 and a first separator 18, and a second diffusion layer 16 may be provided between the second catalyst layer 15 and a second separator 18. The first separator 18 has a fuel gas passage 27 formed therein for supplying fuel gas (hydrogen) to the anode 14, and the second separator 18 has an oxidant gas passage formed therein for supplying oxidant gas (oxygen, usually, air) to the cathode 17. The first and second separators have a coolant passage 26 on opposite sides of the fuel gas passage 27 and the oxidant gas passage 28. The first and second separators 18 further have a fuel gas manifold 30 and an oxidant gas manifold 31 formed therein and fluidly connected to the fuel gas passage 27 and the oxidant gas passage 28, respectively, and a coolant manifold 29 formed therein and fluidly connected to the coolant passage 26. One or more fuel cell 10 constructs a module. As seen by way of example in FIG. 1, number of modules are piled, and electrical terminals 20, electrical insulators 21, and end plates 22 are disposed at opposite ends of the pile of modules. After the pile of modules is tightened between the opposite end plates 22 in a fuel cell stacking direction, the opposite end plates 22 are coupled to a fastening member (for example, a tension plate 24) extending in the fuel cell stacking direction outside the pile of fuel cells, by bolts and nuts 25 to construct a stack of fuel cells 23.
In the PEFC, at the anode 14, hydrogen changes to positively charged hydrogen ions (i.e., protons) and electrons. The hydrogen ions move through the electrolyte membrane 11 to the cathode 17 where the hydrogen ions react with oxygen supplied and electrons (which are generated at an anode of the adjacent MEA and move to the cathode 17 of the instant MEA through a separator) to form water as follows:
At the anode: H2→2H++2e−
At the cathode: 2H+2e−+(½)O2→H2O
The fuel gas passage 27, the fuel gas manifold 30, the oxidant gas passage 28, the oxidant gas manifold 31, the coolant passage 26 and the coolant manifold 29 construct a fluid passage. The fluid passage is sealed so that fuel gas, oxidant gas and coolant are not mixed with each other and do not leak outside.
Japanese Patent Publication No. 2002-289223 discloses that in a fuel cell including a separator made from metal, a gasket made independently of the separator is provided between the separator of a fuel cell and a separator of an adjacent fuel cell to seal between the two fuel cells.
In the fuel cell including the metal separator 18 and a rubber gasket 40, as illustrated in FIGS. 5 and 6, a seal between a fuel cell and the adjacent fuel cell is made by causing a top portion of the rubber gasket 40, which is fixed by heat to the metal separator 18 of the fuel cell, to contact a separator of the adjacent fuel cell. The seal between the separators of the fuel cell disposed on opposite sides of the MEA is conducted by an adhesive 33 provided between the separators.
However, in a fuel cell using a rubber gasket, the following problems exist:    i) A gasket must necessary be provided independently of the separator.    ii) If the gasket is offset from a normal position, leakage may happen.    iii) If the gasket is fixed to the metal separator by heat, a flash is likely to be generated and such flash has to be removed by hand. When the flash is removed, the gasket itself is likely to be damaged in such manner as to cause leakage. If the removed flash adheres to the gasket, the flash may prevent the gasket from contacting a separator of an adjacent fuel cell and in this way lead to leakage.